Focussed electron beam devices, like e.g. electron microscopes for inspecting specimen or electron beam pattern generators for structuring surfaces of a specimen use electron beams because of their spatial resolution, which can be several orders of magnitude higher than the one of optical light beam devices.
A central part of a focussing electron beam device is the electron beam column which generates the electron beam, accelerates the electrons of the beam to a desired energy and focusses the beam. The electron beam is generated by some kind of electron beam source, accelerated by a voltage between the electron beam source and one or several anodes and focussed by focussing components like e.g. electromagnetic lenses. In addition, an electron beam column can comprise other functional components like e.g. apertures or deflection coils, the latter being a means to scan the focus of the electron beam over an area of interest.
FIG. 1 shows schematically a scanning electron microscope (SEM) with an electron beam column 1—1 as an example of a focussing electron beam device known in the prior art. The schematically drawn electron beam column can also be used for an electron beam pattern generator for structuring the surface of e.g. a mask or a silicon wafer or similar applications. The electron beam column 1—1 in FIG. 1 comprises a field emitter beam source 1-2 with an emitter 1-3 with a sharp emitter tip 1-3a and an extracting electrode 1-4. A 1st voltage source 1-7 generates a first voltage V1 between extracting electrode 1-4 and emitter 1-3 that is high enough to enable electrons at the emitter 1-3a to escape into the vacuum 1-0.
The emitted electrons are accelerated towards one of the anodes, i.e. the first anode 1-5, by a second voltage V2 between emitter 1-3 and first anode 1-5. It is generated by the 2nd voltage source 1-8. In this electron beam column the first anode 1-5 is placed directly behind the extracting electrode 1-4 to accelerate the electron beam 1-20 to a sufficiently high energy early on in order to minimize lateral spreading of the beam. Due to the opening 1-5a in the first anode 1-5 a significant fraction of the electron beam 1-20 passes by the first anode 1-5 to enter the focussing components. The electron beam first passes the 1st electron lens 1-10, then the 1st aperture 1-11, then the 2nd electron lens 1-12, then the 2nd aperture 1-13 and then the scan coils 1-14 which bend the electron beam through the final aperture 1-15 toward a desired focus position on the specimen 1-21. The specimen 1-21 is supported by a second anode 1-26. The voltage of the second anode 1-26 with respect to the emitter 1-3 is part of the set of second voltages V2. It is determined by the sum of the voltages of the 2nd voltage source 1-8 and the 3rd voltage source 1-9. The scan coils 1-14 enable a scan of the primary electron beam 1-20 over regions of interest of the specimen 1-21. Certainly, many variations of focussing electron beam devices are known in the art.
The electron beam column shown in FIG. 1 is able to focus the electron beam 1-20 with spot sizes smaller than 10 nm. The current sensing unit 1-24 serves to observe the electron beam current. If the field emitter beam source 1-2 has no other connection to ground than voltage reference 1-23, the current going through the current sensing unit 1-24 must, on average, be equal to the electron beam current of the electron beam 1-20 arriving at the specimen.
Also shown in FIG. 1 is a radiation detector 1-22 detecting the secondary particles 1-25 like secondary electrons, backscattered electrons or X-rays, which are generated by the interaction of electron beam 1-20 with the surface of the specimen 1-21. The secondary particles are used to detect and reconstruct an image of the scanned surface of the specimen.
With increasing resolving power of the focussing electron beam device the time for scanning a given area of a specimen for inspection or for structuring the surface increases. Therefore, to maintain or even increase the throughput of specimens in electron beam devices it has been proposed to use electron beam devices with multiple electron beams in parallel.
For a long time, electron beam sources have been used which use thermal excitation for electron emission instead of electric fields at the emitter. Their main advantages were the stable operation even at moderate vacuum (<10−5 mbar) and their low costs. However, the ever increasing demand for higher spatial resolution and higher throughput has created an interest into cold field emitter beam sources because of their potentially much smaller source size, higher brightness, smaller energy spread and higher life time. The price to be paid for these advantages are a high operational vacuum (<10−9 mbar) and, related to this, problems with the beam current stability. The reason for beam current instabilities is mainly due to the extreme sensitiveness of the electron emission rate to changes of shape and chemical condition of the emitter tip.
The beam current instabilities, however, represent a limitation to imaging resolution or to the homogeneity for structuring the surface of specimens. One solution to the problem of fluctuating electron beam currents is to continually adjust the electron beam currents to the desired current by means of changing the first voltage V1 between emitter and extraction electrode. This, for example, can be realized by replacing the first voltage source by a current source. However, the problem then arises that the electrical potential of the extraction electrode fluctuates, which can deteriorate the focus stability of the electron beam.
Field emitter beam sources have an additional advantage in that they are capable of being integrated onto a semiconductor substrate using micromechanical or microelectronic fabrication techniques. This feature opens up possibilities to miniaturize electron beam columns. Arrays of electron beam columns e.g. can be used to let several electron beams operate in parallel on a specimen in order to increase the throughput for inspecting or structuring a specimen. Such systems have been described e.g. in the U.S. Pat. Nos. 5,969,362 and 6,145,438. In another application many field emitter beam source can be integrated onto a single chip or substrate as described in U.S. Pat. Nos. 5,717,278, 5,828,163, 5,929,557, and 5,990,612. The field emitter beam sources of such devices can be so densely packed that the array of electron beams can be focussed by one electron beam column. Such electron beam devices, e.g. for structuring the surface of a specimen, offer the combined advantage of high spatial resolution with high throughput.
However, the problem of fluctuating beam currents of field emitter beam sources is even more severe when an array of beams is used. In this case the image or the structure on the surface of the specimen would show artifacts showing regions treated by the individually fluctuating electron beams. Such artifacts would limit the resolution of the image or of the structure on the surface of the specimen.